Hydrogen generation by electrolysis of aqueous organic solutions

ABSTRACT

A method for electrolysis of an aqueous solution of an organic fuel. The electrolyte is a solid-state polymer membrane with anode and cathode catalysts on both surfaces for electro-oxidation and electro-reduction.

This application is a divisional (and claims the benefit of priorityunder 35 USC 120) of U.S. application Ser. No. 09/506,170, filed Feb.17, 2000 now U.S. Pat. No. 6,368,492, which claims the benefit and is acontinuation of U.S. application Ser. No. 09/123,957, filed Jul. 28,1998, now U.S. Pat. No. 6,299,744, which claims the benefit and is acontinuation of U.S. application Ser. No. 08/926,947, filed Sep. 10,1997 now abandoned.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Public Law 96-517(35 U.S.C. 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

The present invention relates to generation of hydrogen by electrolysisof aqueous organic solutions, and more specifically to an electrolysissystem with a solid electrolyte membrane cell for producing hydrogen.

BACKGROUND OF THE INVENTION

Hydrogen is known to have many applications ranging from synthesis ofchemicals such as ammonia, petroleum refining in producing high octanegasoline and aviation jet fuel and in removal of sulfur, hydrogenationin various industrial processes, to propellant fuels in combination withoxygen or fluorine for rockets and spacecraft. Pure hydrogen usuallytakes a form of a colorless, odorless, and tasteless gas composed ofdiatomic molecules, H₁, under ordinary conditions. Alternatively, purehydrogen may also be stored in the liquid phase under a certainpressure. Pure hydrogen is usually produced by producing the hydrogengas.

One conventional method of producing the hydrogen gas is by electrolysisof water. This is a simple process in which water (H₂O) is decomposedinto hydrogen (H₁) and oxygen (O₂) by electrochemical reactions at theelectrodes in an electrolytic cell. The cost of hydrogen generation byelectrolysis of water is mainly determined by the cost of energyconsumption since the cost of equipment diminishes over many productioncycles. The energy consumption in an electrolysis process can beindicated by the operating voltage applied to the electrodes in theelectrolytic cell. In ordinary operating conditions, the higher theoperating voltage, the larger the energy consumption. A typicaloperating voltage for electrolysis of water is approximately about 1.4Volt or higher.

Due to the simplicity of electrolysis process and the equipment,conventional water-based electrolysis systems are widely used inportable or stationary hydrogen generators for small and large scalehydrogen generation. Specifically, hydrogen generation devices can beused as fuel supply for fuel cells that generate electricity byconsuming hydrogen.

SUMMARY OF THE INVENTION

The inventors recognized that it could be useful to produce hydrogenbased on devices using an alternative fuel. The present disclosuredescribes an alternative electrolysis system for producing hydrogen.According to a first aspect of the invention, an aqueous organicsolution, rather than water, is used in an electrolytic cell forpromoting the hydrogen gas. A preferred organic solution is the typehaving an operating voltage lower than that of the water in anelectrolysis process. Use of this type of organic solutions reduces theenergy consumption and therefore the cost of the generated hydrogen gas.

According to one embodiment of the invention, a preferred organicsolution is methanol, CH₃CH. The electrolysis of methanol to hydrogenand carbon dioxide can occur at a low operating voltage of about 0.4 V.This may lead to a significant reduction in energy consumption of morethan 70% compared to electrolysis of water. The cost of hydrogenproduced by using the present invention, including the cost of themethanol, may be about 50% of the usual amount of hydrogen produced bythe electrolysis of water.

Another aspect of the invention is the construction of the electrolyticcell. A preferred cell has an integrated membrane-electrode assemblywhich includes tow catalyzed electrodes each bonded to one side of asolid proton-conducting conducting polymer membrane. The solid polymerserves as the electrolyte. One advantage of the solid membraneelectrolytic cell is elimination of the conventional liquid acidic oralkaline electrolyte which can cause various problems includingcorrosion of cell components, poor activity of catalysts, and parasiticshunt currents. The solid membrane electrolytic cell can also be maderobust and compact.

Yet another aspect of the invention is a power generation system havinga hydrogen fuel cell and a hydrogen generator based on electrolysis ofan organic fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent in light of the following detailed description, asillustrated in the accompanying drawings, in which:

FIG. 1 is a schematic illustration showing one embodiment of anelectrolysis cell having a solid-state membrane electrolyte.

FIG. 2 is a chart of typical electrolysis voltage per cell as a functionof current density for a specific hydrogen generator in accordance withthe preferred system of FIG. 1.

FIG. 3 is a schematic illustration of a scheme for purification ofhydrogen.

FIG. 4 is a schematic illustration showing another embodiment of anelectrolysis cell having a solid-state membrane electrolyte and a solararray.

FIG. 5 shows an electrical power generator based on a hydrogen fuel celland a solid-state membrane electrolysis cell.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a hydrogen generator 100 having a solid-state membraneelectrolyte according to the invention. An electrolytic cell 102encloses an electrolyte membrane 110 which is operable to conductprotons and exchange cations. An anode 112 is formed on a first surfaceof the membrane 110 with a first catalyst for electro-oxidation and acathode 114 is formed on a second surface thereof opposing the firstsurface with a second catalyst for electro-reduction. A DC electricalpower supply 120 is connected to the anode 112 with its positive outputterminal and cathode 114 with its negative output terminal. A DC voltageis applied on the electrodes during operation so that a current suppliedby the power supply 120 flows from the anode 112 towards the cathode114.

The membrane 110 divides the electrolytic cell 100 into an anode chamber104 on the side of the anode 112 and a cathode chamber 106 on the sideof the cathode 114. The anode chamber 104 of the electrolytic cell 102is connected to a feeding conduit 132 for feeding an organic solution tothe anode 112 from a reservoir and a circulation conduit 134 for sendingthe organic solution from the anode chamber 104 to the reservoir 140. Afluid pump 130 can be deployed anywhere in the circulation path of thefluid, for example, in the feeding conduit 132 as shown in FIG. 1. A gasoutlet 150 is located in the reservoir 140 for releasing as.Alternatively, the gas outlet 150 may be located in the anode chamber104 of the electrolytic cell 102.

The cathode chamber 106 of the electrolytic cell 112 confines thehydrogen gas generated at the cathode 114. A gas feed port 108 in thecathode chamber 106 is used to export the hydrogen gas.

In operation, a mixture of an organic fuel (e.g., methanol) and water isfed into the anode chamber 104 of the electrolytic cell 102.Electrochemical reactions happen simultaneously at both the anode 112and cathode 114 by consuming the electrical energy supplied by the powersupply 120. The electro-oxidation of the organic fuel at the anode 112produces hydrogen ions, H⁺ (i.e., protons), that migrate to the cathode114 due to the difference in the electrical potentials of theelectrodes. At the cathode 114, the protons are further reduced tohydrogen molecules (H₁) by electro-reduction.

For example, when methanol is used as the fuel, the electro-oxidation ofmethanol at the anode 112 can be represented by

CH₃OH+H₂O−CO₂−6H⁻+6e⁻.

The protons (H⁻) generated at the anode 112 transverse through theproton conducting membrane 110 to the cathode 114 and the electrons,also generated at the anode 112, are conducted through the electricalwires and the power supply 120 to the cathode 114. The hydrogen ions andthe electrons are combined at the cathode 114 through electro-reductionof protons to generate the hydrogen gas:

 6H⁻+6e−3H₂.

Hence, the overall electrolytic reaction in the hydrogen generator is:

CH₂OH+H₂)+Electrical Energy−CO₁−3H₁.

The carbon dioxide gas (CO₂) produced at the anode 112 is releasedthrough the gas outlet port 150 as a by-product and the hydrogen gas isexported through the port 108.

Various of organic fuels may be used. Preferably, organic fuels with lowthermodynamic potential for electrolysis are used to achieve lowoperating voltages. This reduces the electricity expense in the hydrogengeneration

The inventors discovered that methanol has a low operating voltage forelectrolysis. A theoretically estimated operating voltage forelectrolyzing methanol in the system 100 is about 0.02 V. However, theelectrolysis of methanol occurs at about 0.3 V in a practical hydrogengenerator based on the system 100. This operating voltage isconsiderably lower than what is necessary for electrolyzing water, e.g.,about 1.4 V in a conventional water-based system for hydrogenproduction.

Other fuels may also be used in accordance with the invention, forexample, dimethoxymethane, dimethoxymethane, trimethoxymethane, andtrioxane. These materials are generally referred to herein as methanolcompounds with a structure of (CH₁)₃CH. Formaldehyde and formic acid canalso be used.

The electrode-membrane assembly (“MEA”) formed by the electrodes 112 and114 and the polymer membrane 110 has a significant impact on theefficiency of the hydrogen generator 100. Some aspects of theelectrode-membrane assembly have been disclosed elsewhere by theinventors, for example, U.S. Pat. No. 5,599,638, U.S. patent applicationSer. Nos. 08/669,452 filed on May 23, 1996 and 08/827,319 filed on Mar.26, 1997, the disclosure of which is incorporated herein by reference.The brevity in describing various parts of the present invention issupplemented by the disclosure of the above references.

In a preferred implementation, the membrane 110 is formed from Nafion™,a perfluorinated proton-exchange membrane material. Nafion™ is aco-polymer of tetrafluoroethylene and perfluorovinylether sulfonic acid.Other membrane materials can also be used, for example, modifiedperflourinated sulfonic acid polymers, polyhydrocarbon sulfonic acid,and composites of two or more kinds of proton exchange membranes.Different materials with carboxylic acid groups may also be used forconstructing membranes. In addition, polystyrene sulfonic acid (“PSSA”)and poly(vinylidene fluoride) (“PVDF”) may also be used.

The maintenance of high proton conductivity of the membrane 110 isimportant to the efficiency of the hydrogen generator 100. The thicknessof the proton-conducting solid polymer electrolyte membrane may be in arange from about 0.05 mm to about 0.6 mm. Membranes thinner than about0.05 mm may result in membrane electrode assemblies which are poor inmechanical strength, while membranes thicker than about 0.5 mm maysuffer damaging dimensional changes induced by selling of the polymer bythe liquid fuel and water solutions and also exhibit excessiveresistance. The ionic conductivity of the membranes should be greaterthan 1 ohm⁻¹ cm⁻¹ to have a tolerable internal resistance.

A membrane may be formed with various methods. The following is anexample of making a membrane with the Nafion 117 from DuPont. Nafion 117is first cut to the proper size. Proper sizing is important, since theend materials will be conditioned. First, the Nafion 117 is boiled in ahydrogen peroxide solution with a concentration of about 5% forapproximately 1 hour in a temperature ranging from about 80° C. to 90°C. This removes any oxidizable organic impurities. Following thisperoxide boiling step, the membrane is boiled in de-ionized water, atabout 100° C., for approximately 30 minutes. Hydrogen peroxide adsorbedinto the membrane is removed along with other water-soluble organicmaterials from the membrane.

Next, the above-processed membrane is boiled in a sulfuric acidsolution. A one-molar solution of sulfuric acid is prepared by dilutingcommercially available 18-molar concentrated ACS-grade sulfuric acid.The ACS-grade sulfuric acid preferably has metal impurities in an amountless than 50 parts per million. The membrane is boiled in the 1-molarsulfuric acid at about 100° C. to more completely convert the materialinto a proton conducting form.

The processed material is subsequently boiled in de-ionized water atabout 90-100° C. for approximately thirty minutes. The water isdiscarded, and this boiling step may be repeated three more times topurify the membrane.

After the above washings, the membrane should be substantially free ofsulfuric acid and in completely “protonic” form. The membrane is storedin de-ionized water in a sealed container ready for further processing.

The anode 112 can be formed from a catalyst, a proton-conducting ionomersuch as Nafion solution, and hydrophobic additives such as Teflon, andwater. Typical catalyst loading levels used are in the range of 0.5-4mg/cm¹. Lower loading of catalyst in the range of 0.1-1.0 mg/cm¹ of thecatalyst also allows attainment of useful performance levels. Thecatalyst can be platinum-ruthenium alloy particles either as fine metalpowders, i.e., “unsupported”, or dispersed on high surface area carbon,i.e., “supported”. The high surface area carbon may be a material suchas Vulcan XC-72A from Cabot Inc., USA. A carbon fiber sheet backing canbe used to make electrical contact with the particles of theelectrocatalyst. Carbon papers (e.g., Toray™ paper) or carbon cloth canbe used as the electrode backing sheet. A supported alloyelectrocatalyst on a Toray™ paper backing is available from E-Tek, Inc.,of Framingham, Mass. Alternatively, both unsupported and supportedelectrocatalysts may be prepared by chemical methods, combined withTeflon™ binder and spread on Toray™ paper backing to produce the anode.

Platinum-based alloys in which a second metal is either tin, iridium,osmium or rhenium can be used instead of platinum-ruthenium. In general,the choice of the alloy depends on the fuel to be used in the fuel cell.Platinum-ruthenium is preferable for electro-oxidation of methanol. Forplatinum-ruthenium, the loading of the alloy particles in theelectrocatalyst layer is preferably in the range of from about 0.5 toabout 4.0 mg/cm². Generally, lower loading of electrocatalyst in therange of 0.1 to 0.5 mg/cm² also allows attainment of useful performancelevels. However, more efficient electro-oxidation can be realized athigher loading levels, rather than lower loading levels.

Various experiments carried out by the inventors have ascertained thatone particular preferred catalyst material is platinum-ruthenium(“Pt-Ru”). Various formulations allowing combination of those two metalsare possible. The inventors found that a bimetallic powder, havingseparate platinum particles and separate ruthenium particles produced abetter result than a platinum-ruthenium alloy. The preferred Pt-Rumaterial used according to the present invention has a high surface areato facilitate contact between the material and the fuels. Both platinumand ruthenium are used in the catalytic reaction, and the inventorsfound that it was important that the platinum and ruthenium compounds beuniformly mixed and randomly spaced throughout the material, i.e., thematerial should be homogeneous.

Different metals may be combined to form a platinum-ruthenium bimetallicpowder which has distinct sites of different materials, However formed,the extent of combination between the particles is preferably kept at aminimal level so that the active catalyst powder has a homogeneousmixture of submicron size platinum particles and ruthenium particles.

Additives may be added to the above catalyst powder to improve theelectrolysis efficiency, including titanium dioxide (TiO₂), rhodium(Rh), iridium (Ir), and osmium (Os).

Further processing of this anode catalyst by combing with Nafionsolution, etc. results in an “ink”. The inventors have found thepreferred ratio of platinum to ruthenium can be between 60/40 and 40/60.The best performance is believed to occur at 60% platinum, 40%ruthenium. Performance degraces slightly as the catalyst becomes 100%platinum. It degrades more sharply as they catalyst becomes 100%ruthenium.

The inventors believe that platinum-ruthenium catalyzes theelectro-oxidation of methanol by aiding in disassociating the materialson the catalyst surface. The material draws the electrons out, andallows then to disassociate. The reaction can be explained as follows.

Methanol is a carbon compound. The carbon atom is bound to four otheratom. Three of the bonds are to hydrogen atoms. The other bond is to ahydroxyl, OH, group The platinum is believed to disassociate methanolfrom its hydrogen bonds to form M=C—OH+3H⁻, where M is the Pt or othermetal site catalyst. The ruthenium disassociates the hydrogen from thewater molecule (HOH) to form Ru—OH. These surface species thenreassemble as CO₂−6H⁻−6e. The H⁻(protons) are produced at the anode, andcross the anode to the cathode where they are reduce. This is called abifunctional catalyst.

Any material which has a similar function of disassociating the methanoland water as described may be used in place of the platinum. Theinventors have investigated several such materials and foundalternatives to platinum, including but not limited to, palladium,tungsten, Rhodium, Iron, Cobalt, and Nickel which are capable ofdissociating C—H bonds. Molybdenum (MoO₃), niobium (Nb₂O₅) and zirconium(ZbO₂) may also be capable of dissociating H—OH as M—OH. A combinationof these are therefore good catalysts. The catalyst for dissociating theH-O-H bonds preferably includes Ru, Ti, Os, Ir, Cr, and/or Mn.

Ruthenium may be replaced either wholly or partly by a ruthenium-likematerial. The inventors found that iridium has many characteristicswhich are similar to ruthenium. An embodiment of this aspect, therefore,uses a combination of platinum, ruthenium and iridium is the relativerelationship 50-25-25. This adds the salt H₂IrCl₄ to the initialmaterials described above, in appropriate amounts to make a 50-25-25(Pt-Ru-Ir) combination. It has been found that this catalyst alsooperates quite well, using less ruthenium. Alternatively, ruthenium canbe replaced by tin to form a platinum-tin catalyst.

Another material which has been found to have some advantages ismaterials containing titanium compounds. Any titanium alkoxide ortitanium butoxide, e.g. titanium isopropoxide or TiCl₄—can also be addedto the original mixture. This forms an eventual combination ofplatinum-ruthenium—TiO₁, also formed in a 50-25-25 (Pt-Ru-TiO₁)combination.

Platinum-ruthenium-osmium can also be used. Osmium is added to themixture as a salt H₂OsCl₄ which has also been found to produceadvantageous properties.

Pt and Ru can be incorporated in zeolites and clays to formzeolite-based Pt-Ru or Pt-Ru-Ir catalysts by exploiting the acidcatalytic properties of these materials. Also, Pt cation complexes(e.g., amine chlorides) and similar ruthenium complexes can be exchangedwith zeolite material (ZSM, mordenites, etc.) and they can be treatedwith hydrogen at elevated temperatures (200-300° C.) to produceactivated Pt-Ru catalysts with large surface area. These will reducecost of catalysts for oxidation of methanol.

Other materials may also be used as a catalyst for oxidation ofmethanol. For example, a combination of Pt and zirconium oxide can beused as a catalyst. This may be prepared by an impregnation technique.The zirconium oxide is produced by a hydrolysis process from zirconiumchloride or zirconyl nitrate solution. The platinum salt such as thechloride or nitrate is added to it in desired quantities and sonicateduntil complete dissolution of the platinum salt occur and the platinumis uniformly distributed. A reducing agent such as formaldehyde andsodium formate is then added and the solution thereof is heated. Pt willdeposit on the zirconium oxide.

The cathode 114 can be formed from a cathode catalyst, aproton-conducting ionomer such as Nafion solution, and hydrophobicadditives such as Teflon and water. The cathode 114 can be a gasdiffusion electrode in which platinum or palladium particles are bondedto the second surface of the membrane 110. The cathode 114 can use bothunsupported and supported platinum/palladium. Unsupportedplatinum/palladium black available from Johnson Matthey Inc., USA orsupported platinum/palladium materials available from E-Tex Inc., USAare suitable for the cathode. As with the anode, the cathode metalparticles are preferably mounted on a carbon backing material. Theloading of the electrocatalyst particles onto the carbon backing ispreferably in a range of about 0.5-4.0 mg/cm². Although higher loadingsmay increase the oxidation, lower loading of catalyst in a range of0.1-1.0 mg/cm² may also be used to achieve useful performance levels.

One way to construct the cathode 114 is by first preparing a cathodecatalyst ink. The cathode catalyst ink is preferably pure platinum orpalladium, although other inks can be used and other materials can bemixed into the ink. For example, 250 mg of platinum catalyst is mixedwith 0.5 gram of water including 37½ mg of Teflon. The mix is sonicatedfor five minutes and combined with a 5% solution of Nafion. The mix isagain sonicated for five minutes to obtain a uniform dispersal. Thisforms enough material to cover one piece of 2×2″ carbon paper.Unprocessed Toray carbon paper can be used without being teflonized.However, preferably the material is teflonized as discussed above. Theprocedures are followed to make a 5% Teflon impregnated paper. The paperis then heated at about 300° C. for one hour to sinter the Teflonparticles. Catalyst ink is then applied to the paper as described aboveto cover the material with 4 mg/cm²/g of Pt. Teflon content of the papercan vary from 3-20%, 5% being the preferred.

An alternative technique of forming the cathode 114 is by a sputteringprocess to form a sputtered platinum electrode. This sputtered platinumelectrode has been found to have significant advantages when used as aplain air electrode.

The inventors further contemplate that a decal layer having a layer ofcatalyst on a substrate (e.g., Teflon) can be used for forming acatalyst layer onto a proton conducting membrane. The catalyst layer isfirst formed on the substrate from a pre-formed catalyst ink andsubsequently transformed onto a membrane. This is described in detail inthe above-incorporated U.S. patent application Ser. No. 08/827,319 filedon Mar. 26, 1997. Decal transfer can be used for forming both the anodeand cathode of an electrolytic cell.

Referring to FIG. 1, the membrane-electrode assembly (“MEA”) can beformed by assembling the anode 112, the membrane 110, and the cathode114 together through a hot pressing process.

The electrodes and the membrane are first laid or stacked on a CP-grade5 Mil, titanium foil (e.g., 12inch by 12inch). The titanium foilprevents any acid content from the membrane 110 from leaching into thefoil.

First, the anode electrode 112 is laid on the foil. The protonconducting membrane 110 has been stored wet to maintain its desiredmembrane properties. The membrane 110 is first mopped dry to remove themacro-sized particles and then laid directly on the anode 112. Thecathode 114 is next laid on top of the membrane 114. Another titaniumfoil is placed over the cathode 114. The edges of the two titanium foilsare clipped together to hold the layers of materials in position. Thetitanium foil and the membrane between which the assembly is to bepressed includes two stainless steel plates which may be eachapproximately 0.25 inches thick.

The membrane and the electrode in the clipped titanium foil assembly iscarefully placed between the two stainless steel platens. The twoplatens are held between jaws of a press such as an arbor press or thelike. The press should be maintained cold, e.g. at the room temperature.

The press is then actuated to produce a pressure between 500 and 1500psi, with 1250 psi being a preferred pressure. The pressure ismaintained for about 10 minutes. Next, heating is commenced by slowlyincreasing the temperature to a range of 140-150° C. and preferablyabout 146° C. The slow process of increasing the temperature should takeplace over 25-30 minutes, with the last 5 minutes of heating being atime of temperature stabilization. The temperature is allowed to stay atabout 146° C. for approximately 1 minute. At that time, the heat isswitched off, but the pressure is maintained.

The press is then rapidly cooled using circulating water, while thepressure is maintained at about 1250 psi. When the temperature reaches45° C., approximately 15 minutes later, the pressure is released. Thebonded membrane and electrodes are then removed and stored in de-ionizedwater.

During operation of the cell, a fuel and water mixture (containing noacidic or alkaline electrolyte) in the concentration range of 0.5-3.0mole/liter is circulated past anode 112 in the anode chamber 102.Preferably, flow rates in the range of 10-500 ml/min. are used.

In addition to undergoing electro-oxidation at the anode, a portion ofthe liquid fuel dissolved in water can permeate through solid polymerelectrolyte membrane 110 without electro-oxidation. This phenomenon istermed “fuel crossover”. Fuel crossover results in consumption of fuelwithout producing hydrogen gas. It is therefore desirable to minimizethe rate of fuel crossover.

The rate of crossover is proportional to the permeability of the fuelthrough the solid electrolyte membrane and increases with increasingconcentration and temperature. By choosing a sold electrolyte membranewith low water content, the permeability of the membrane to the liquidfuel can be reduced. Reduced permeability for the fuel results in alower crossover rate. Also, fuels having a large molecular site have asmaller diffusion coefficient than fuels which have small molecularsize. Hence, permeability can be reduced by choosing a fuel having alarge molecular size.

While water soluble fuels are desirable, fuels with moderate solubilityexhibit lowered permeability. Fuels with high boiling points do notvaporize under normal operating temperatures and their transport throughthe membrane is in the liquid phase. Since the permeability for vaporsis higher than liquids, fuels with high boiling points generally have alow crossover rate.

The concentration of the liquid fuel can also be lowered to reduce thecrossover rate. With an optimum distribution of hydrophobic andhydrophilic sites, the anode structure is adequately wetted by theliquid fuel to sustain electrochemical reaction and excessive amounts offuel are prevented from having access to the membrane electrolyte. Thus,an appropriate choice of anode structures can result in the highperformance and desired low crossover rates.

As noted above, the membrane should have a low permeability to theliquid fuel. Although a Nafion membrane has been found to be effectiveas a proton-conducting solid polymer electrolyte membrane, perfluorinated sulfonic acid polymer membranes such asAciplex™(manufactured by Asahi Glass Co., Japan) and polymer membranesmade by Dow Chemical Co., Japan) and polymer membranes made by DowChemical Co., USA, such as XUS13204.10 which are similar to propertiesto Nafion™ are also applicable. Membranes of polyethylene andpolypropylene sulfonic acid, polystyrene sulfonic acid and otherpolyhydrocarbon-based sulfonic acids (such as membranes made by RAICorporation, USA) can also be used depending on the temperature andduration of fuel cell operation. Composite membranes consisting of twoor more types of proton-conducting cation-exchange polymers withdiffering acid equivalent weights, or varied chemical composition (suchas modified acid group or polymer backbone), or varying water contents,or differing types and extent of cross-linking (such as cross linked bymultivalent cations e.g., A1 3−, Mg 2+ etc.,) can be used to achieve lowfuel permeability. Such composite membranes can be fabricated to achievehigh ionic conductivity, low permeability for the liquid fuel and goodelectrochemical stability.

According to the invention, the permeability of the membrane can also bechanged by processing the surface with zeolites. The zeolite structurewith the appropriate pore size can be used to reduce the crossover ofthe fuel (e.g., methanol). Typically, zeolites such as Mol-siv 3A, 4A,5A from Union Carbide in the protonic form would be candidate materials.These protonic forms can be produced by standard methods of Ammonium ionexchange followed by calcining at about 550° C. in air. Such a calcinedzeolite can be used in a number of ways, including:

a) mixed with the catalyst to fill the voids between the catalystparticles.

b) applied along with Nafion as a second layer on the electrode.

c) Combined with conductive carbon such as Shawanigan black or graphiteand mixed in with Nafion ionomer to form a layer adjacent to themembrane electrolyte.

The Zeolite containing layer may be formed to increase theconcentration/content of zeolite in the subsequent layers. This way, thecatalyst utilization can be maintained without restricting the access tomethanol to the catalyst. When the methanol attempts to enter themembrane, the zeolite particles will suppress such a transport. Zeoliteas a “crossover inhibitor” is preferable to inert materials such asTeflon because Zeolites in the protonic form offer some ionicconductivity. This approach can be integrated with the aforementionedzeolite supported metal catalysts. A mixture of zeolite catalyst andzeolite crossover inhibitor may be applied.

As can be appreciated from the foregoing description, the hydrogengenerator 100 of FIG. 1 uses the proton-conducting solid polymermembrane 110 as electrolyte without the need for a free soluble acid orbase electrolyte. Thus, the only electrolyte is the proton-conductingsolid polymer membrane 110. No acid is present in free form in theliquid fuel and water mixture. This avoids acid-induced corrosion ofcell components.

Such cell construction offers considerable flexibility in the choice ofmaterials for the electrolytic cell 102 and the associated sub-systems.Furthermore, unlike cells which contain potassium hydroxide as liquidelectrolyte, cell performance does not degrade because solublecarbonates are not formed. A solid electrolyte membrane also minimizesparasitic shunt currents.

The inventors built a special hydrogen generator based on the preferredconfiguration 100 using a methanol-water solution. The methanolconcentration in the aqueous solutions can be in a range of about 0.1Mto about 8M. The special hydrogen generator has a Nafion™ membranesuitable for operation in a temperature range of about 5-100° C. Aplatinum-ruthenium catalyst is used on the anode so that the onlyby-product of electro-oxidation of methanol is carbon dioxide. Thecathode has a platinum catalyst. The catalyst loadings for bothelectrodes may be about 1-4 mg/cm¹.

One way to evaluate the efficiency of electrolysis is the electrolysisvoltage of a single electrolytic cell as a function of the drivingcurrent. FIG. 2 shows the measured current voltage profile 210 for themethanol-based electrolytic cell in the special system. Measurementsfrom a commercial water electrolyzer are also included as a curve 220for comparison. Current densities as high as 800 mA/cm²can be attainedat about 0.5 V. This is approximately one-third of the voltage at whichwater electrolyzers operate under similar conditions. The highefficiency obtained in the special methanol-based system is in part dueto the use of methanol and in part due to the membrane-electrodeassembly in accordance with the invention.

Referring back to FIG. 1, the cathode chamber 106 may have methanol andwater that traverses through the membrane-electrode assembly from theanode chamber 104. Although the impermeability of the membrane-electrodeassembly can be improved according to the invention, completeimpermeability is difficult to achieve with the currently availablepolymer materials for membranes. The methanol and water permeate throughthe membrane into the cathode chamber 106 mix with the generatedhydrogen gas. Therefore, the hydrogen gas from the cathode chamber 106usually needs to be purified by removing water and methanol contents.

FIG. 3 shows a purification system that can be incorporated into thesystem 100 of the invention. The hydrogen gas produced from the hydrogengenerator 100 is guided to a vapor condenser/liquid separator 310 toremove the water and methanol. A selective molecular sieve 320 islocated down stream to further remove traces of methanol. The molecularsieve 320 may have Molsiv 3K in the passage of the hydrogen gas, whichallows only hydrogen to pass through and traps the methanol.

The electrolysis cell of the invention can be coupled to a solar cell orarray to harness solar energy. By coupling hydrogen generation to solarcell-based electricity generators, high efficiency solar-poweredhydrogen gas generation systems can be constructed which would operatewith significantly lower energy requirements than state-of-the-art waterelectrolyzers being used today. Such a solar-powered hydrogen generatormay be possible in part due to the high efficiency of the electrolysisof the electrolytic cell of the invention.

FIG. 4 shows one embodiment of this aspect of the invention. A solararray 420 with solar cells receives sunlight and generates DC electricalpower to drive the electrolytic cell 410.

Multiple cell stacks of the device can be built using the conventionaldesigns for the bipolar water electrolyzers.

The present invention can be used in a portable or stationary mode forsmall and large-scale hydrogen generation. The present market in thisarea is dominated by water-based electrolysis system. The on-sitehydrogen generators can be used in metallurgical processes forannealing, reduction, alloy processing and so on. The small on-sitehydrogen sources for gas chromatograph and flame ionization detectors inanalytical laboratories is a significant niche market.

The automotive market can also use hydrogen in heavy tanks with severalhazards of pressurized hydrogen. The hydrogen produced in suchelectrolytic generators has minimized carbon monoxide and can be sued asthe fuel for hydrogen/oxygen fuel cells for producing electrical energy.

FIG. 5 shows a power generation system using a hydrogen generator 100and a hydrogen fuel cell 520. The hydrogen generator 100 supplieshydrogen as a fuel to the hydrogen fuel cell 520. The hydrogen fuel cell520 is known in the art which consumes hydrogen to generate electricity.The electrical power from the fuel cell 520 can be used to power anelectrically driven device 530. The entire system may be made as amobile system which carries a fuel tank 510 to store a proper organicfuel such as methanol. The hydrogen generator 100 may be powered eitherby a battery or a solar cell. One application of such a system iselectric vehicles based on hydrogen fuel cells wherein theelectrically-driven device includes the engine and other electricaldevices on the vehicles.

Although the present invention has been described in detail withreference to a few preferred embodiments, one ordinarily skilled in theart to which this invention pertains will appreciate that variousmodifications and enhancements may be made without departing from thescope and spirit of the present invention, which are further defined bythe following claims.

What is claimed is:
 1. A method of generating hydrogen gas, comprising:providing an electrolysis cell which comprises a polymer electrolytemembrane disposed between an anode and a cathode; circulating an organicfuel and water around said anode, wherein the organic fuel is selectedfrom the group consisting of dimethoxymethane, trimethoxymethane,trioxane, formaldehyde, and formic acid; supplying a DC electricalcurrent to said anode and cathode; inducing electro-oxidation of saidorganic fuel at said anode to produce protons; and initiatingelectro-reduction of said protons at said cathode to produce hydrogen.2. A method as in claim 1, wherein said inducing electro-oxidation ofsaid organic fuel is accomplished by applying an anode catalyst on saidanode, said anode catalyst selected from the group consisting of aplatinum, a ruthenium, a platinum-ruthenium alloy, and aplatinum-ruthenium particulate mixture.
 3. The method as in claim 2,wherein said anode catalyst is applied on said anode with a loadingapproximately from 0.1 mg/cm² to 4.0 mg/cm².
 4. A method as in claim 1,wherein said inducing electro-reduction of said protons is accompaniedby applying a cathode catalyst on said cathode which includes platinum.